I don't know if this is a bug, or an unintended consequence of the new feature to show topics from subforums when viewing a forum.

If a user is viewing a forum which has sub forums, the add topic button doesn't allow the user to post in the sub forums.

So the add-topic button behaves different from say viewing posts in a category which allows a user to post in all forums and sub forums.

I think expected behavior would be to all the add-topic button on a forum to post in either that forum or sub-forums of that forum (depending on user access to post in the various forums)